A
helical (corkscrew) shape permits the spiral artery to extend into
the thickening stroma of the proliferative endometrium
and also, later, to retract back into the stratum basalis when the stratum
functionalis is eventually sloughed off. Because of its corkscrew shape,
each spiral artery typically appears as an aligned cluster of profiles
(asterisks) deep in the endometrium.